This invention relates to time slot interchangers which are used in communication systems employing time division multiplex (TDM) switching techniques. A time slot interchanger (TSI) receives and temporarily stores information contained in time slots in a TDM system so that the information received in one time slot can be subsequently retransmitted during another time slot. Thus, a TSI functions as a switch that routes communications between one party or path associated with a first time slot and another party or path associated with a second time slot.
Although control of a TSI can be implemented in hardware or software, the software implementations offer substantial flexibility as compared with a hardware implementation. In a typical software implementation, random access memory (RAM) stores received information contained in a first time slot at a predetermined address. The RAM location containing this information is subsequently accessed and the information transmitted during another time slot. Each received time slot is associated with a separate RAM address location. Information in an output time slot is obtained by reading the information stored at the RAM address corresponding to an input time slot associated with the output time slot. Encoded digital signals, such as created by pulse code modulation (PCM) techniques, may be utilized to carry the information.
A common application of a TSI is in a telephone switching system where it may cooperate with a space switching module to provide electronically switchable channels for connecting call originating subscribers with called subscribers. A predetermined number of time slots comprise a frame which is utilized for timing and synchronization purposes. A TSI must be capable of handling at least the number of time slots which may be concurrently utilized.
A large telephone switching system places a substantial burden on a TSI utilized in the system. The number of time slots that can be concurrently handled by a TSI limits the maximum number of subscribers which can be handled by the switching facility. For example, a switching facility with a maximum capacity of 15,000 calls would require a corresponding TSI capable of handling 30,000 concurrent time slot assignments since one outbound channel and one inbound channel are required to support each duplex conversation. In order to minimize delays through the switching system, it is desirable to operate a TSI at very high clock speeds. However, the transmission of signals at high clock speeds places demands on the transmission lines or data buses utilized to carry the time slot data to and from a TSI. For large switching systems, physical constraints make it difficult, if not impossible, to construct the TSI function on a single, reasonably sized, printed circuit board or module. The use of separate modules to implement a TSI further aggravates the transmission difficulties encountered at high clock speeds.
Prior art TSI facilities capable of handling a large number of time slots have been constructed as separate modules with input and output slot information communicated to each module in parallel over either a single bus or separate input and output buses. While such implementations have generally been successful, increased capacity demands and the resulting desire for higher clock speeds and faster data transmission capabilities present problems. It is difficult to minimize the undesirable transmission problems, such as transmission line reflections, when parallel bus connections are utilized for high speed data transmission, especially when the number of modules served by the parallel bus can change. In a growable system, adding or removing TSI modules causes a parallel bus to experience a change in impedance. As the number of modules connected to a bus increase, the capacitive load presented by the modules increase making the bus more difficult to drive. In order to provide for resynchronization of the data distributed by a parallel bus, a large number of wide elastic storage devices may be required. Thus, there exists a need for an improved TSI which minimizes these difficulties.